Conditioning of cellulose fiber for conversion into cellulose acetate



Patented Apr. 1938 CONDITIONING or cELnuLo'sE FIBER FOB CONVERSION m'ro- CELLULOSE acn- TATE George A. Richter, Berlin, N. 11., assignor to Brown Company, Berlin, N. 11., a corporation of Maine No Drawing. Application May z, 1936,

' Serial No. 77,612

r: Claims. (c1. zso-ioi) This invention relates to the conditioning .of cellulose fiber for conversion'into cellulose acetate and more particularly to a conditioning treatment that activates and otherwise improves the cellulose for the acetylating reaction. While not limited thereto, the conditioning treatment of the present invention is of especial usefulness when applied to cellulose fiber which is more or. less refractory toward acetylatingireagents.

Wood pulp which has been purified to high alpha cellulose content in alkaline liquors are apt to acquire passivity toward acetylating reagents; and the stronger the alkaline liquor, the more passive is the pulpapt to become. Indeed, wood pulp which has been refined in alka line liquor of sufficient strength to be mercer izing, say, caustic soda solution'of about 18% or greater strength, generally becomes so passive as to require an activating treatment -.;of some kind before it can be acetylated with the desired smoothness or uniformity of reaction. Again, sheeted cellulose fiber, such as pulpboard and waterleaf paper, is' sometimes poorly responsive to acetylation even when it is finely shredded prior to acetylation; and its poor responsiveness may be attributable largely to its overdried condition rather than to the nature of its cellulose fiber. In this connection, it might be observed that it is quite desirable that cellulose fiber to be acetylated contain very little moisture,.preferably not much' more than about 2% of its dry weight, so as to avoid undue dilution of the acetylating reagents. However, in drying pulpboard or waterleaf'paper to the desired low residual moisture content, as on the usual steam-heated drier drums of a. papermaking machine, it is frequently the case that in getting-'maximum output on the machine, the sheet is broughtto a bone-dry condition, 1. e. overdried, particularly on its surface, Just as it is undesirable to have much more than about 2% water present in the cellulose to be acetylated, so a bone-dry condition therein is to be avoided, as bone-dryness may spell'a shrunken and dense fiber structure resistant to '.acetylation.

In accordance withthe present invention, cellulose fiber to be acetylated is first put through an activating treatment with a mixture of hydrogen peroxide and acetic acid, the mixture preferably containing asit's acetic acid component a fraction of the glacial acetic-acid subsequently to enter into the acetylatingreaction. I have found that such a mixturenot'only greatly improves the acetylatability of the fiber but also enhances the color and clarity of the usual solutions prepared from the acetylated product for such purposes as artificial silk and mm manufacture.- Moreover, I have foundthat such a mixture can render cellulose fiber of the refractory character ,hereinbefore described satisfactorily. acetylatable by the usual acetylating reagents. Ihe conditioning or activating treatment hereof presents the important advantage that there is no'need for washing or otherwise removing the activating reagents, inasmuch as the hydrogen peroxide leaves no undesirable residues in the fiber and theacetic acid can take part in the'subsequent acetylation. While various percentages of hydrogen peroxide may be used in admixture with acetic acid for the activating treatment hereof, it is preferable to use' aqueous hydrogen peroxide solution. -in such amount and in such highly concentrated condition together with glacial acetic acid as to dispense with the -step of dehydrating or drying of the activated fiber, preparatory to acetylation. Unlike the hydrogen peroxide solution usually employed for medicinal or antiseptic purposes,

which is of about 3% hydrogen peroxide content,

the hydrogen peroxide solution used herein is preferably of much greater strength. Although. it is difiicult and expensive to preparevery strong hydrogen peroxide solutions, aqueous solutions of about hydrogen peroxide content are;

commercially available at a cost reasonable for 30 the purpose hereof, wherefore, I may advantageously'use such solutions. To be sure, such lsolutions=contain more than twice as much water as hydrogen peroxide, but because only a very. small percentage of hydrogen peroxide, based 5 on dry fiber, is suflicient when admixed' with glacial acetic acid to cause the desired activation or improvement of cellulose fiber, it becomes possible to use such aqueous hydrogen peroxide solutions in the activating treatment and to add 40,

so little water to the fiber as to avoid practically any drying or dehydratiomot the activated fiber prior to its acetylation.

An illustrative example of the'practice of the present invention may be substantially as follows. Pulpboard or waterleaf paper of a moisture content of about 1% or less and umatisi'actorily .responsive to 'acetylating reagents may constitute the raw material. Besides being bonedry on its surfaces, such sheeted fiber may be so composed of wood pulp or cotton fiber, which,

on account of purification in an'alkaline liquor,; has taken on passivity toward acetylating agents. For instance, the sheeted fiber may be composed of white, highly purified wood pulp, 66

been brought to an alpha cellulose content upwards of about 96% in a caustic soda solution of 18% or greater strength. Such'a sheet may be wetted with hydrogen peroxide-acetic acid mixture prepared by mixing 3 to parts by 1 weight of hydrogen peroxide solution of about 30% strength with 97 to 90 parts by weight of glacial acetic acid; and about parts of such a mixture may be substantially uniformly applied to the surfaces of about 100 parts by weight of the sheeted fiber. The application of the mixture may be effected as by spraying the sheet surfaces therewith or by transferring the mixture to the sheet surfaces from the periphery of a so-called "kissing" roll. The treated a sheet containing less than 2% added water, about 0.2 to 0.6% added hydrogen peroxide, and about 17% acetic acid, based on the dry weight of fiber, may be kept for about two to eight or even-more hours at about 20 to 40 C. to effect the desired activation of the cellulose, after which period it is desirable to add to the sheet as such or after it has been disintegrated more .of the glacial acetic acid later to form part of the acetylating reagents, for instance, as much as about 400%,

based on the dry weight of the fiber, together integrated or shredded, if this has not already.

been done, and admixed with acetic anhydrlde and more acetic acid to produce the usual acetylating bath or mixture, the last addition of glacial acetic acid giving a total glacial acetic acid content of, say, about 100%, based -on the dry weight of the fiber. As the fiber is stirred in the acetylating reagents, it undergoes smooth and uniform acetylation so that the original -thick fibrous suspension gradually becomes a substantially clear cellulose acetate solution from which the cellulose acetate is. precipitated and recovered as customarily.

The foregoing example lends itself to wide modification as to the concentration of the aqueous hydrogen peroxide solution in the activating mixture applied to the sheeted fiber and/or as to the amount of the mixture infused into the fiber. If such concentration and/or amount is such as to produce in the sheet a total water content of much greater than about 2 to 3%, say 5% or more, basedcn the dry weight of fiber, the treated sheet may be ex-- posed to warm air currents for a sufllcient period of time to reduce the water content to about 2% and thus to avoid undesirable dilution of the acetylating reagents in which the fiber is finally acetylated. It is generally; preferable to keep the hydrogen peroxide concentration in the activating mixture as high as possible compatible with the'incorporation into the sheeted fiber of activating mixture in amount to impart to they sheet a moisture content not much in excess of 2% of the dry weight of fiber. After keep- After the second portion of acetic acid' specifically, mercerized wood pulp which has glacial acetic acid to appear in the acetylating reagent together with the catalyst of such reagent; and this larger portion not. only ensures a uniform distribution of the peroxide throughout the sheet, but assists in the attainment of a final sheet uniformly and well activated for the acetylating reaction. Thus, the sheet may be sprayed or otherwise treated continuously with the activating mixture as it is coming from a papermaking machine or from a roll accumulation, whereupon it may be accumu'-- lated as a roll and kept for a period; or the sheet wetted with the activating mixture may be exposed in festoon form to the air either at room or elevated temperature. The sheeted fiber may then be' impregnated with a large additional amount of glacial acetic acid containing the catalyst of acetylation. as by running the sheet continuously through a bath of such acid or by dipping separate sheets thereinto. After keeping the acid-soaked sheet fiber at a suitable temperature for a substantial period of time, for

instance, at 40 C. for four or more hours, it may be acetylated, as hereinbefore described.

It is possible to apply the conditioning or activating treatment hereof to bulk or shredded the glacial acetic acid may be reduced so as not to incorporate an excessive amount of hydrogen peroxide or water into the fiber. A very uniform and eflective' treatment of bulk fiber with the activating mixture can be realized by suspending the fiber in many times its own weight of the activating mixture; and, after a substantially uniform fiber suspension has been prepared, as much as possible of the activating mixture may be removed by a centrifuge, squeezing, or similar extracting operation. The latter treatment of the bulk fiber may be accompanied by the addition to the fiber of so much more than the approximately 2% water content desired in fiber intended for acetylation that provision must be made to evaporate or otherwise remove from the fiber water in excess of about 2%. This may, as

already indicated, be accomplished by exposing the centrifuged or pressed fiber to the dehydrating action of warm air currents.

It is possible that the mixture of hydrogen peroxide and acetic acid herein used for the purpose of conditioning or activating cellulose fiber for acetylation owes its eifectiveness to a partial reaction between the acetic acid and the hydrogen peroxide productive of 'peraoetic acid. Irrespective of whether peracetic acid is or is not present in the activating mixture, cellulose fiber is greatly improved in its acetylatability by such a mixture and, when of a refractory acetylatable state.

I claim:

1. A process which comprises treating cellulose fiber with a mixture of hydrogen peroxide and acetic acid and then acetylating the treated fiber in the presence of not much more than about 2% water, based on the dry weight of cellulose fiber.

nature, is transformed thereby toa satisfactory 2. A process which comprises treating ceilulose fiber with a mixture of concentrated aqueous acid and then acetylating the treated fiber in.

hydrogen peroxide solution and glacial acetic the presence of not much more than about 2% water, based on the dry weight of cellulose fiber.

3. A process which comprises treating cellulose fiber with a mixture of concentrated aqueous hydrogen peroxide solution and glacial acetic acid, adding more glacial acetic acid to the fiber,

and acetylating the fiber in the presence of not much more than about 2% water, based on the dry weight of cellulose fiber.

4. A process which comprises treating cellulose fiber with a mixture of glacial acetic acid and a smaller amount of aqueous hydrogen peroxide solution of about strength and then acetylating the treated fiber in the presence of not much more than about 2% water, based on the dry weight of cellulose fiber.

5. A process which comprises treating cellulose fiber with a mixture of glacial .acetic acid and a smaller amount of aqueous hydrogen peroxide solution of about 30% strength, adding '7. A process which comprises treating a sub-- stantially dry sheet of cellulose fiber with a mixture of glacial acetic acid and concentrated aqueous hydrogen peroxide solution in amount to add to ,the sheet not much more than about 2% water, based on the dry weight of fiber, adding more glacial acetic acid to the sheet,and acetylating the treated fiber constituting said sheet.

8. A process which comprises treating cellulose fiber with a mixture of acetic acid and aqueous hydrogen peroxide solution of such water content and in such amount to associate with the fiber considerably more than 2% water, based on the dry weight of fiber, evaporating from the fiber its water content in excess of about 2%. and acetylating the resulting fiber.

9-. A process which comprises treating cellulose fiber with a mixture of acetic acid and aqueous hydrogen peroxide solution of such water content and in such amount to associate with the fiberconsiderably more than 2% water, based on the dry weight of fiber, evaporatingirom the fiber its water content in excess of about'3%,

of such hydrogen peroxide."

adding glacial acetic,acid to the fiber, andacety-' lating the resulting fiber.

10. As a step-product especially adapted for conversion into cellulose. acetate, cellulose fiber treated with a mixture composed-substantially 01! hydrogen peroxide, acetic acid, and only a small percentage ofwater, based on the dry weight of fiber. g a

11. As a step-product especially adapted for conversion into cellulose acetate, cellulose fiber treated with amixture of hydrogen peroxide-and glacial acetic acid and containing not much more than about 2% water, based on the dry weight of'fiben' 12. As a step-productespecially adapted for conversion into cellulose acetate, wood pulp of high alpha cellulose content treated with a mixture of hydrogen peroxide and acetic acid and containing not much more than about 2% moisture, based on the dry weight oi pulp. 13. As a stop-product especially adapted for conversion into cellulose acetate, white mercerized wood, pulp of an alpha cellulose content upwards of about 96% treated with a mixture of hydrogen peroxide and acetic acidand containing not much more than about 2% moisture,

based on thedry weight of pulp.

14. As a step-product especially adapted for conversion into cellulose acetate, a sheet of cellulose fiber treated with amixture of aqueous hy-' drogen peroxide solution of about 30% strength and a larger amount of glacial acetic acid but containing not much more than about 2.% water, based on the dry weight off the fiber.

15. As a step-product-especi'ally adapted for conversion into cellulose acetate, a sheet of wood pulpof high alpha cellulose content treated with 1 a mixture of aqueous hydrogen peroxide solution of about 30% strength and a larger amount of glacial acetic acid but containing not much more than about 2% water, based on the dry weight of pulp.

16. As a step-product conversion into cellulose acetate, a sheet of white mercerized wood pulp of an alpha cellulose content upwards' of about 96% and treated with a mixture of aqueous hydrogen peroxide solution of about 30%-strength and -a larger amount or glacial acetic acid but containing not much more than about 2% water, based on the dry weight of pulp. 1

1'7. A process which comprises in more aqueous hydrogen peroxide solutio into sub-1 stantially dry cellulose while'keepingthe water content of the cellulose at not much more than about 2%, based on the dry weight or celluespecially adapteddor Y lose, and acetylating the cellulose in the presence GEQRGE A. RICHTER. 

